This invention relates generally to switching power supplies and, more particularly, to two-stage switching converters with transformer separation. One area in which the invention finds particular utility is in connection with high end battery chargers.
The increasing demand for size reduction of virtually all electronic appliances is difficult to meet with presently available switching power supplies. Several features inherent in devices of relevant technology militate against achievement of the desired goal. For example, the whole converter must typically be designed to handle a relatively wide range of input voltage, resulting in low efficiency. Moreover, reducing the size of magnetic and filtering elements require high switching frequencies, with concomitant high converter losses. High losses are inherent for high frequency operation due to the necessity of dissipating the energy stored in the leakage inductance of the transformer to avoid large voltage spikes across the main transistor. Also, implementation of isolation of the feedback signal is costly and bulky, and the whole converter has to be redesigned if there are changes in the load voltage requirements.
Switching power supplies presently employed in high end battery chargers typically use a single stage off-line converter, most commonly a flyback converter. Rectified and filtered line voltage is applied to the converter which provides isolation from the mains and regulates the output voltage. To regulate the charging current, the converter needs a feedback signal from the output. The feedback is normally provided by an optocoupler which passes the signal to the control providing isolation. A typical circuit is depicted in FIG. 1. The present invention is directed to overcoming one or more of the problems or disadvantages associated with the present state of the art.
The present invention, in the embodiments described hereinafter, offers the following features and advantages:
1. Compensation for variations in input voltage is effected in two different stages of the circuit, permitting design of each part for a smaller regulation range, resulting in more efficient circuits.
2. The reduced regulation range in each stage also results in reduced voltage and current stress in the components, permitting the use of lower cost components to provide the same performance.
3. Changes in load voltage requirements can be accommodated with redesign of only the second stage.
4. Output regulation is performed in the second stage, which is isolated from the mains, whereby no isolation of the feedback signal is required.
5. Efficiency is further enhanced by utilizing the secondary converters as a synchronous rectifier with less losses of the diode used in conventional relevant technology.
6. The first stage, in a preferred embodiment, is a half-bridge, resonant, self-oscillating circuit, operating with zero voltage switching (ZVS) with much lower switching losses, recovery of the leakage energy, and clamping of the voltage of the switches to the voltage after the line rectifier.